The present invention generally relates to geographical information systems and, more particularly, to maps of the type bearing a graphic representation of cartographic information, broken down into a large number of elementary areas with which are associated respectively a large number of indexes.
At the present time, two different approaches are used in cartography, between which there is generally no interpenetration.
The older approach, consisting in the use of maps bearing a graphic representation, is still very widely used, both in professional applications and in popular applications. The maps give global information that is directly accessible and familiar to all potential users. They remain easy to handle even when produced in large formats, in the order of one square meter. When they are made using modern printing techniques, which have now been thoroughly mastered, very fine lines can be produced on them: many maps available at low prices are produced using polychromy by superimposing frames, each formed by a matrix of dots with a pitch of 100 .mu.m or less (down to 25 .mu.m), and with a dot positioning accuracy of approximately 10 .mu.m.
Counterbalancing these advantages, the conventional graphic representation type map has certain limitations. In particular, the amount of useful information that can be inscribed on a map without detriment to its legibility is limited. This often leads to the greater part of the information being placed in an appendix. The text and graphic information are generally interlinked by means of a simple squaring system, whereof each square is identified, by means of a letter-figure pair or by a number. Such squaring gives only an approximate indication and, in addition, obliges the user to refer to indications provided in the margins of the map (see document FR-A-2 639 452).
The data processing approach to geographical representation has essentially consisted, hitherto, in defining an image, dot by dot, in the form of a table, in a memory. Even if use is made of representational algorithms in a simplified form, the necessary information takes up very large volumes of memory, which can amount to as much as several hundreds of millions of bytes for a single map. In addition, existing visual display devices are very far from offering the same degree of resolution as the conventional graphic maps. A professional quality high resolution monitor displays an image in the form of pixels each of which can be likened to a square with sides of approximately 300 .mu.m.